Effect

In all generations, Curse acts differently depending on whether the user has the Ghost type or not.

Ghost-type Pokémon

The user will lose half of its maximum HP (rounded down) and put a curse on the target. If the user has less than half of its maximum HP remaining, the move will execute fully but cause the user to faint. A cursed Pokémon will lose ¼ of its maximum HP at the end of each turn. The curse will remain until the Pokémon switches out or the battle ends. This move is not blocked by Protect or Detect, but is blocked by Crafty Shield.

The user does not lose HP if Curse misses (due to a move with a semi-invulnerable turn), fails (due to the target already being affected by Curse), or is blocked (by Crafty Shield).

Non-Ghost-type Pokémon

The user's Speed stat will drop by one stage and its Attack stat and Defense stat will rise by one stage each. Despite being an effect acting on the user, this move will fail if there are no opponents on the field.

Generation II

If a cursed Pokémon defeats its opponent, it will not take curse damage on that turn.

Generations III-IV

If a cursed Pokémon defeats its opponent, it now takes curse damage on that turn.

If the user changes type before using Curse, Curse will obey the user's type at the time of the move's execution. In a Double Battle, if the user gains the Ghost type before Curse executes, Curse will prioritize the opponent directly opposite the user as its target.

Generation V

Curse is now a Ghost-type move. Its effects remain the same.

In a Triple Battle, if the user gains the Ghost type before Curse executes, Curse will prioritize the opponent directly opposite the user as its target.

Generation VI

This move effect may be in need of research. Reason:What happens when there is no ally and the user gains Ghost without Protean, or if a foe uses Follow Me? What happens if the user gains Ghost before using Curse in a Single or Horde Battle?You can discuss this on the talk page.

In a Double Battle, if the user is not already Ghost-type and becomes Ghost-type before executing Curse (due to Color Change or Trick-or-Treat), Curse will now target the user's ally if the user is on the right side (from its Trainer's perspective), or a random opponent if the user is on the left side (from its Trainer's perspective). In a Triple Battle, if the user is not already Ghost-type and becomes Ghost-type before executing Curse (due to Color Change or Trick-or-Treat), Curse will now target a random adjacent opponent.

If the user is not already Ghost-type and becomes Ghost-type due to Protean upon executing Curse, the user will use the Ghost-type Curse on itself (regardless of Follow Me or Rage Powder); this cannot be blocked by Crafty Shield.

Trivia

Curse's differing effects are a play on its Japanese name, のろい Noroi, which can mean either "curse" (呪い) or "dull/slow" (鈍い), depending on the kanji used. This is why it bestows an actual curse when used by Ghost-type Pokémon but lowers Speed when used by non-Ghost-type Pokémon.

In Generation II, TM03, which contains Curse, is given away only at night by a man telling the following story:

"Let me recount a terrifying tale... Once upon a time, there was a little boy who was given a new Bicycle... He wanted to try it right away... He was having so much fun that he didn't notice the sun had set... While riding home in the pitch-black night, the bike suddenly slowed! The pedals became heavy! When he stopped pedaling, the bike began slipping backwards! It was as if the bike were cursed and trying to drag him into oblivion! ... ... Shrieeeek! The boy had been riding uphill on Cycling Road! ... Ba-dum ba-dum! For listening so patiently, you may take this—TM03! TM03 is Curse. It's a terrifying move that slowly whittles down the victim's HP."